The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
An ever-increasing number of relatively inexpensive, low power wireless data communication services, networks and devices have been made available over the past number of years, promising near wire speed transmission and reliability. Various wireless technology is described in detail in the 802 IEEE Standards, including for example, the IEEE Standard 802.11a (1999) and its updates and amendments, the IEEE Standard 802.11n, and the IEEE draft standards 802.15.3, and 802.15.3c now in the process of being finalized, all of which are collectively incorporated herein fully by reference.
As one example, a type of a wireless network known as a wireless personal area network (WPAN) involves the interconnection of devices that are typically, but not necessarily, physically located closer together than wireless local area networks (WLANs) such as WLANs that conform to the IEEE Standard 802.11a or the IEEE draft standard 802.11n. Recently, the interest and demand for particularly high data rates (e.g., in excess of 1 Gbps) in such networks has significantly increased. One approach to realizing high data rates in a WPAN is to use hundreds of MHz, or even several GHz, of bandwidth. For example, the unlicensed 60 GHz band provides one such possible range of operation.
In general, antennas and, accordingly, associated effective wireless channels are highly directional at frequencies near or above 60 GHz. As a result, the distance separating a pair of communicating devices has a significant impact on the data rate that the pair of communication devices can support. Further, when multiple antennas are available at one or both communicating devices, an efficient beam pattern allows the devices to better exploit spatial selectivity of the wireless channel and, accordingly, increase the data rate at which the devices communicate. Generally speaking, beamforming or beamsteering creates a spatial gain pattern having one or more high gain lobes or beams (as compared to the gain obtained by an omni-directional antenna) in one or more particular directions, with reduced the gain in other directions. If the gain pattern for multiple transmit antennas, for example, is configured to produce a high gain lobe in the direction of a receiver, better transmission reliability can be obtained over that obtained with an omni-directional transmission.
Thus, when devices are separated by a relatively large distance, and especially when an efficient beamforming pattern has not yet been applied, the data rate at which these device communicate may measure only tens of Mbps. In some systems, devices communicate at this data rate when the physical layer of the protocol stack is in the so-called control mode operation (“control PHY”). Generally speaking, control PHY of a transmission system corresponds to the lowest data rate supported by each of the devices operating in the transmission system. Devices transmit and receive control PHY frames to communicate basic control information such as beacon data or beamforming data, for example. On the other hand, when beamformed devices are separated by a short distance, a data rate in excess of 1 Gbps can be established. Beamformed devices generally transmit data at the higher data rate using “normal PHY” data units.
In wideband wireless communication systems that operate in the 60 GHz band, packets transmitted via a communication channel generally include a PHY preamble to provide synchronization and training information; a PHY header to provide the basic parameters of the physical layer such as length of the payload, modulation and coding method, etc.; and a PHY payload portion. A PHY preamble consistent with the IEEE 802.15.3c Draft D0.0 Standard, for example, a synchronization field (SYNC) that includes several repetitions of a certain spreading sequence to indicate the beginning of a block of transmitted information, a start frame delimiter (SFD) field to signal the beginning of the actual frame, and a channel estimation sequence (CES) to carry information for receiver algorithms related to automatic gain control (AGC) setting, antenna diversity selection, timing acquisition, coarse frequency recovery, channel estimation, etc.
In such systems, the PHY preamble takes up some of the bandwidth available in the communication link. Further, when the PHY payload of a packet is relatively small, the proportion of the bandwidth occupied by the PHY preamble of the packet can be relatively large.